A variety of gases are used in manufacturing semiconductor devices such as LSIs. These gases contain impurities. These impurities have been suspected to have an adverse influence on the characteristics of the LSIs. Hence, it is demanded that the gases be as pure as possible This demand grows stronger, along with the increase in the integration density of LSIs. To meet this demand, a high-accuracy and reliable analysis of gases is required
The techniques commonly used for analyzing such gases for determining the impurity contents thereof are: gas chromatography (GC), gas chromatography-mass spectroscopy (GC-MS), and Fourier transformed infrared spectroscopy (FTIR). The detection limit of these techniques are, however, 1 to 10 ppb at best. In view of this, these analytical techniques cannot be said to determine the impurity content of the gases as sensitively as is required in the manufacture of LSIs.
Furthermore, in order to conduct a successful quantitative analysis on a particular species in a sample gas by a non-absolute method such as mass spectroscopic method, it is necessary to make a calibration curve by using standard gas mixtures containing the species to be analysed. Theses standard mixtures can be prepared by diluting a gas mixture of a known high concentration of the impurity, with a diluent gas. The concentration of the species in the final mixture should be preferably in the same range as that in the sample gas to be analyzed. When the concentration of the sample gas is very low, the following factors determine the accuracy of the analysis
a. Detection limit of the analytical instrument PA0 b. Purity of the diluent gas PA0 c. Mixing techniques PA0 with a back pressure regulator connected at the end of the pipe delivering the low concentration gas mixture, thus avoiding about any contamination of the pipe. PA0 with the analysis apparatus itself which is sometimes able to control itself said pressure (e.g. when the analysis is carried out at atmospheric pressure) PA0 with a pressure regulator connected at the output of the analysis apparatus, thus providing no contamination of the low concentration gas mixture.
When the detection limit of the analytical instrument is in lower orders, it is difficult to obtain factors b and c, both in the comparable orders.
Recently, a highly sensitive analytical instrument, which is called atmospheric-pressure ionization mass spectrometer (APIMS) has been developed. This instrument can determine the contents of molecular species down to 10 ppt. Therefore, it has become desirable to produce standard gas mixtures in the low concentration range.
Low-concentration standard gas mixtures, which is used as a calibration gas in analyzing extremely pure gases, could be produced by diluting a high-concentration standard gas in one or a plurality of stages. In order to produce a low-concentration standard gas, continuously at a desired flow rate and pressure, the two-stage dilution method, for example, is employed In this method, first the high-concentration standard gas is diluted with a diluent gas of the same kind of the sample gas to a predetermined medium lower concentration, then most of this medium concentration mixture is discarded, and the remaining fraction of said gas is further diluted with the diluent gas. The concentration of the species within this low concentration, final standard gas must be controlled accurately. For this purpose, various devices such as mass flow controllers, pressure regulator, . . . must be used to control the flow rates of the material gas and the diluent gases. As soon as low concentrations should be made of species that make part of our natural environment, the use of regulation devices like mass flow controllers, pressure regulators, etc. . . . would give serious limitations on the lower limits that can be achieved.